In many optical scanning systems a final moving mirror is used between the final converging lens and the image plane. Such a moving mirror could be used for the relatively slow y-scan after a rotating prism and other optics have generated a relatively fast x-scan. A major problem associated with such scanning systems is that, in general, as the final mirror rotates, the point of focus of the scanning light beam will follow a curve which does not lie in the image plane. For example, if the mirror pivots or rotates about a fixed axis lying in the plane of the mirror, the point of focus will follow a circular path of fixed scan radius with the result that the beam will be substantially out of focus at the two ends of the scan if it is in focus at the center of the scan.
A common solution to the aforedescribed "field curvature" problem is to make the scan radius very large, thereby reducing the necessary scan angle and the deviation from flatness of the focus path. For a given image resolution, this solution requires large, slow and often expensive optical components. Another solution to the "field curvature" problem is to shift the scan mirror during its pivotal movement in a direction transverse to the axis of its pivotal movement. This shifting movement of the mirror is so controlled and predetermined, for example, by a cam surface, that for each angular setting of the mirror the total length of the light path to the image plane is the same for all angular settings or positions of the mirror. For accurate mirror movement in a direction substantially transverse to the axis of the pivotal movement of the mirror, the latter solution requires that the cam have a precision ground non-linear surface which is often expensive and difficult to produce.